The present invention relates to tinoxide thin film, a prototype of an n-type semiconductor, of which optical band gap is about 3.5 eV, mainly used for transparent electrodes or for detecting reductive, inflammable gases (H.sub.2, CH.sub.4, C.sub.3 H.sub.8, etc.). More specifically, the present invention relates to tinoxide thin film which satisfies nonstoichiometry/stoichiometry, respectively, the preparation thereof, and a gas-detecting sensor prepared by the use of such tinoxide thin film.
Conventional semiconductor-type gas detecting sensors are roughly divided into sintering type, thick film type and thin film type. Most inflammable gas sensors commercially available at present are bulk-type sensors prepared by a sintering process where powdery raw material is molded under pressure and then heat-treated at high temperature. However, bulk type sensors and thick film type sensors have lower sensitivity to gases compared to thin film type sensors, and are difficult to be mass-produced and require higher costs for production.
On the other hand, thin film type sensors are useful in that they can be easily prepared as microsensors, and have high sensitivity to gases.
Recently, research for developing microsensors using thin film material, particularly processes for preparing novel thin films in which rare elements can be easily doped have been extensively proceeded, because incorporation of rare elements in thin films is essential to increase the performance of sensors.
As thin films for manufacturing gas detecting sensors, tinoxide thin film is most widely used, and such tinoxide thin film should be homogeneous SnO.sub.2 type so as to allow a dopant to exhibit the performances when the doping element is added to increase the electric and/or chemical characteristics of the tinoxide thin film.
For manufacturing the tinoxide thin film, processes for manufacturing ceramic thin films by the use of chemical vapor deposition(CVD) such as plasma enhanced (PE) CVD and metal organic (MO) CVD are known. However, it is almost impossible to control the thickness, orientation, crystallinity, density, and micropores of the thin films, using these known processes.
When physical vapor deposition process such as DC or RF sputtering, ion beam sputtering or ion assisted deposition(IAD) are used, it is possible to manufacture thin films varying organo metallic materials, bias potentials, compositions of the target etc. However, in these cases, it is difficult to control the thin film properties.
Takagi et al. prepared a SnO.sub.2 thin film in a high vacuum chamber containing oxygen, using an ionized cluster beam deposition(ICBD) process [H. Takaoka, K. Matsubara and T. Takagi, Proc. 4th Symp. on Ion Sources and Ion Application Technol. Kyoto (1980), p143]. According to this process, however, the temperature of the substrate plate was as high as 400.degree. C., and the obtained thin film was polycrystalline tin oxide thin film having large amount of Sn, SnO, Sn.sub.2 O.sub.3 etc. admixed therein as well as SnO.sub.2.